1. Field of the Invention
The present invention relates generally to fluid purification and, in particular, to a method and assembly for the photocatalytic oxidation of contaminants within fluid streams.
2. Description of the Related Art
One of today's most pressing societal problems is environmental pollution. The Earth's atmosphere is contaminated by hazardous organic pollutants such as solvents, alcohols, dyes, and fuel oils. Such contaminants pose serious health risks. As a result, purification systems are needed to clean air and water to healthy levels in both the home and the work place. Purification systems also serve to permit contaminating factories to comply with environmental regulations.
In recent years, photocatalytic oxidation has emerged as a generally effective method of purifying fluids such as air and water. This method involves directing a flow of a contaminated fluid into contact with an oxidation catalyst that is simultaneously exposed to electromagnetic radiation. Pollutants contained within the fluid are adsorbed onto the surface of the oxidation catalyst. As a result, the pollutants are oxidized and thereby decomposed into environmentally innocuous components such as water and carbon dioxide. One catalyst that has been used is anatase titanium dioxide (TiO.sub.2). The absorption of ultra violet light energy causes the TiO.sub.2 to become highly reactive.
Early photocatalytic oxidation techniques were unsuitable for incorporation into conventional HVAC systems because they were limited to relatively slow flow rates or were applicable only for liquid flowstreams. For example, U.S. Pat. No. 5,045,288 to Raupp et al. teaches a method wherein a contaminated fluid flows vertically upward through a reaction chamber containing a catalyst bed of loose anatase titanium dioxide particles resting upon filter paper. Ultra violet light from a source outside of the reaction chamber is exposed onto the outer periphery of the catalyst bed, producing the desired oxidation of contaminants. One limitation of the Raupp system is that the presence of the filter paper and the large catalyst bed causes a relatively high pressure loss within the fluid. The filter paper, although sufficiently porous to permit fluids to slowly flow through it, tends to restrict the flow. Thus, the fluid pressure decreases as it passes through the filter. Above the filter and downstream thereof, the large catalyst bed further restricts the flow, decreasing the pressure even more. Such pressure loss limits the fluid flowrate and, consequently, the overall rate of purification as well.
Another limitation is that only those catalyst particles on the outer edges of the catalyst bed are exposed to the ultra violet light. Such limited surface area of light exposure results in limited overall oxidation, non-uniform oxidation rates within the flowstream, and non-use of the catalyst particles in the center of the catalyst bed, which are not exposed to the light.
U.S. Pat. Nos. 5,163,626 and 5,308,458 to Urwin et al. teach a method wherein a flow of contaminated liquid is exposed to ultra violet light as it flows over a horizontal spinning disc utilized to agitate the liquid. According to a first method, anatase titanium dioxide is mixed within the contaminated liquid to produce the desired contact between the contaminants and the catalyst. According to a second method, the catalyst is coated onto the spinning disc to produce the desired contact. The disc is coated by immersing the disc within an aqueous solution of the catalyst and then baking the disc. The immersion/baking cycle is repeated 7 to 15 times.
There are several limitations of the Urwin system. One limitation is that it cannot effectively be used for gas flowstreams, such as air. Regarding the first method, it is difficult if not impossible to mix the anatase titanium dioxide particles within a gas, since the particles generally are not light enough to be carried by the gas. Regarding the second method, a gas flowstream is not desirable because most of the gas will not come into contact with the disc surface. Rather, most of the gas will flow above the disc and avoid being oxidized by the titanium dioxide. Another limitation of the Urwin system is that it is relatively complex and expensive to manufacture. For example, the inlet tube through which the liquid flows onto the spinning disc also spins along with the disc. The utilization of moving parts makes it more difficult to maintain a leak-free environment and necessitates frequent replacement of motors and other parts. Another limitation of the first method in particular is that the process necessitates the further step of filtering the titanium dioxide particles from the purified liquid. Another limitation of the second method in particular is that the repeated immersion/baking process is relatively time-consuming, yet produces a highly non-uniform coating and a relatively weak bond between the catalyst and the disc.
More recent photocatalytic oxidation methods involve less expensive, passive filters which are more suited for use in conventional HVAC systems. Such filters have relatively large fluid passageways therein so that a fluid stream may pass through the filter without significant pressure loss. Also, such filters do not have moving parts like the spinning disc of the Urwin system, which may complicate the design and necessitate frequent replacement of such parts. The filter is typically manufactured by coating an adhesive binding material, such as a polymer, epoxy, or other binder, onto an inert substrate. The binding material may either be intermixed with a catalyst or the catalyst may be coated onto the binding material after the latter is applied to the substrate. A limitation of such filters is that ultra violet radiation tends to bum away the adhesive material, causing the catalyst to flake off of the substrate.
One example is U.S. Pat. No. 5,564,065 to Fleck that teaches a purification system comprising a reaction chamber filled with a fine fibrous material, such as fiberglass. A powder form of anatase titanium dioxide catalyst is coated onto the fibrous material by first applying an adhesive liquid onto the material and then spraying the catalyst thereto. The liquid adhesive is applied by immersing the fibrous material in a bath of the liquid adhesive and then removing the fibrous material. As the catalyst is sprayed onto the material, catalyst particles stick to the adhesive liquid coating, forming a layer of the catalyst on the fibrous material. Ultra violet light is generated by a light source in the center of the chamber.
Similarly, Japanese patent Application No. 10-238799 teaches a filter comprising an aluminum corrugated honeycomb substrate coated with a colloidal silica type binder containing anatase titanium dioxide catalyst. The catalyst is first mixed with the binder. The binder is then coated onto the substrate to form the filter. Ultra violet light is provided by a nearby light source.
Several characteristics of such filters limit their effectiveness. One limitation is that, as mentioned above, the ultra violet light tends to bum away the adhesive material that carries the catalyst. In operation, these filters lose catalyst particles relatively quickly and must be frequently replaced. Another limitation is that use of an adhesive binding material often results in an uneven coating of the catalyst on the filter, resulting in a waste of unusable catalyst. Another limitation is balancing the need to maximize contaminant contact with the catalyst with the need to minimize pressure drop access the filter. Prior art photocatalytic oxidation systems do not strike the balance very well, resulting in either high contact and high pressure drop or low pressure drop, but low contact. A further limitation is that with some systems only a very limited surface area of the catalyst is exposed to the ultra violet light, resulting in a lower overall oxidation rate, non-uniform oxidation, and waste of the unexposed catalyst.
Although prior art photocatalytic oxidation systems are generally effective, there is a need to improve purification rates by providing increased catalyst surface contact area and by increasing the portion of the catalyst surface that is directly exposed to electromagnetic radiation. There is also a need to increase system efficiency by minimizing the loss of catalyst through general use, resulting in less shutdowns for filter replacement.